1. Field of the Invention
The present invention relates to an optical fiber type temperature distribution measuring apparatus for measuring a temperature distribution in an electric power facility, plants of various types, or the like by utilizing Raman scattering light, and in particular, to such an apparatus in which the spatial resolution (or distance resolution) is improved.
2. Description of the Prior Art
Recently, as described in an article "Raman scattering light utilized distribution type temperature sensor" (magazine "SENSOR TECHNOLOGY", Vol. 9, No. 7, May 1989, pp. 30 to 34), an optical fiber type temperature distribution measuring apparatus for simultaneously measuring temperatures at a multiplicity of positions by using a single optical fiber has been proposed. This apparatus utilizes a phenomenon in which the intensity ratio between a Stokes' line and an anti-Stokes' line which are two components of Raman scattering light changes sensitively depending on a temperature of an optical fiber. In the measurement, a light pulse is transmitted into the optical fiber, and a time (hereinafter referred to as a delay time) until Raman back scattering light returns to a transmitting end of the optical fiber is measured to determine a position at which the scattering light is generated. On the other hand, a temperature of the optical fiber at the position, that is, the position at which the optical fiber is installed is determined from the intensity ratio. Furthermore, by detecting the Raman back scattering light from respective positions along the optical fiber on the time division basis, the temperatures at respective positions along the optical fiber, that is, a temperature distribution along the optical fiber can be obtained. The principle of the measurement in this apparatus is illustrated in FIG. 5, and a waveform of the Raman scattering light is shown in FIG. 6, and a relationship between intensity ratio and temperature is shown in FIG. 7.
Specifically, as shown in a block diagram in FIG. 4, an optical fiber 2 is installed along an object 1 to be measured in an electric power facility, a plant or the like, and a light pulse 18 is transmitted into the optical fiber 2, within a measuring section 3, from a pulse semiconductor laser 5 which is driven by a pulse driving circuit 4. Subsequently, Raman back scattering light 19 from each position along the optical fiber 2 is received in the measuring section 3, and a Stokes' line and an anti-Stokes' line which are two components of the Raman back scattering light are split or separated by two types of interference filters 7 and 8 in an optical branching filter 6, and the intensities of the split Stokes' line and anti-Stokes' line are respectively photoelectrically converted by first and second avalanche photodiodes (APDs). Then, the intensities of these two components are A/D converted in a high speed averaging processing unit 11, and the A/D converted intensities are respectively stored in a memory at locations respectively corresponding to delay times. After all the Raman back scattering light 19 is returned from the optical fiber 2, a light pulse 18 is again transmitted into the optical fiber 2, and the detection of Raman back scattering light 19 is carried out, and the obtained intensities are stored by adding to the respective previously stored intensitites in the locations of the memory. After repeating these operations a predetermined number of times (for example, several thousands of times), the intensity values stored in each of the locations of the memory are divided by the number of times of the repetition to obtain an average value. The purpose of this processing for averaging is to prevent a measurement error from being introduced because of the very weak Raman back scattering light. Thereafter, in the high speed processing unit 11, the intensity ratio is obtained for each of the positions on the basis of the average intensity values of the Stokes' line and the anti-Stokes' line, and the obtained intensity ratios are delivered to a data processing unit 12. In the data processing unit 12 temperature distribution information is obtained on the basis of the intensity ratio at each of the positions along the optical fiber 2. The temperature distribution information is displayed on a screen of a display 13. In this respect, in obtaining the temperature from the intensity of ratio between the Stokes' line and the anti-Stokes' line, a map prepared beforehand by experiments and calculations is used.
However, in such a prior art optical fiber type temperature measuring apparatus, the following problems are involved.
In the prior art apparatus mentioned above, the temperature of the object 1 to be measured is measured as an average value in each segment corresponding to a light pulse width. For this reason, in order to measure the temperature distribution accurately, it is necessary to enhance the spatial resolution by narrowing the light pulse width and by shortening a time width for enabling time division (sampling time interval). However, to narrow the light pulse width means a reduction of data which is averaged, and this naturally results in a deterioration of the accuracy of temperature measurement. Considering these situations, the practical light pulse width, that is, the spatial resolution, has been selected to be about 1 meter. Accordingly, the prior art apparatus cannot be applied to measure an object which requires a spatial or distance resolution which is less than 1 m. In this case, a conventional apparatus employing a thermocouple had to be used.